Various types of data storage devices have been developed including high capacity devices that have multiple recording surfaces. A conventional magnetic or optical storage device is typically manufactured as three separate assemblies, the medium, the heads and the controller, which are integrated as a single storage device during one of the final steps in the production process.
A conventional medium assembly includes a plurality of double sided disks arranged in a stack on a central spindle typically rotated by a motor. Each side of each disk is usually plated with a magnetically or optically sensitive coating to form a recording surface for information which is organized in sectors spaced along concentric or spiral tracks on each of the recording surfaces.
The head assembly is used for recording and retrieving the information stored in a sector. Generally the head assembly includes at least one read/write head for each of the surfaces. Each head is typically mounted on a positioning arm which is linked to a servo positioning mechanism capable of moving all heads in unison in a radial or lateral direction over the recording surfaces. Some storage devices may include multiple head assemblies for a single medium assembly.
The controller assembly is used for managing the information flow between the computer system and the medium. A conventional controller assembly may include one or more microprocessors and memories to perform the complex tasks of address translation, head positioning, data transfer, data correction and performance optimization.
Various environmental influences, faulty components parts, contamination and production problems may render a read/write head or surface defective during the manufacturing process, decreasing the production yield or process margin. As a result of these problems, the manufacturers of storage devices have taken precautions to detect and minimize such defects. Typically, the medium, head and controller assemblies are extensively tested before they are put together to form a storage device. However, unintentional contact between the head and surface in the final assembly may cause additional damage to the surface or the head. Also, a complete functional test of the device is not possible until the storage device is fully assembled. Performance testing after assembly may detect additional faulty devices that may need to be scrapped or repaired, further diminishing the manufacturing process margin.
Several approaches or techniques have evolved which partially tolerate limited damage to the recording surface. These techniques allow the use of a faulty device having minimal surface defects without disassembly and repair. A first technique, for very small physical surface defects, is to provide the controller with an error detection and correction capability which can mathematically reconstruct bursts of erroneous data by using correction codes stored with the data. A second technique, used if the error rate exceeds the capabilities of the error correction codes, simply marks the sector containing the surface defect as invalid. A substitute sector within a predesignated spare sector area is provided by the manufacturer to record the information of the damaged sector. A third technique, for larger defects, provides each track with reserve recording space. With this technique, the sectors in the track containing a surface defect are shifted or displaced around the defect in the track. A fourth technique invalidates an entire track and redirects the information to a substitute track elsewhere on the surface.
A disadvantage of conventional correcting techniques is that in avoiding damaged surface areas the access time to the information is increased. Yet another disadvantage of the above techniques is that they only allow the use of faulty devices where the defects are limited to the medium.
The process margin of conventionally manufactured high capacity multi-surface devices may yield only a fractional number of devices where all surfaces and heads are fully functional. More typically, in a substantial number of devices, at least one or more surfaces and/or heads are found to be of marginal or substandard performance.
Therefore, it is desirable that a storage device be constructed with a surface and head reallocation mechanism which improves the yield of the manufacturing process without unduly decreasing the storage capacity of the storage device or requiring disassembly, repair, and reassembly of defective storage devices.